NOVELTY - Oxidative coupling of methane to generate hydrocarbon compounds containing at least two carbon atoms (C2+ compounds), involves: (a) mixing a first gas stream comprising methane with a second gas stream comprising oxygen to form a third gas stream comprising methane and oxygen; and (b) performing an oxidative coupling of methane (OCM) reaction using the third gas stream to produce a product stream comprising one or more C2+ compounds.    USE - The methods and system are useful for the oxidative coupling of methane to generate hydrocarbon compounds containing at least two carbon atoms (C2+ compounds); and for producing at least one C2+ alkene (all claimed).    ADVANTAGE - The methods and system can be used in the energy-efficient, and effective conversion of methane to higher chain hydrocarbons, such as hydrocarbon compounds with two or more carbon atoms (C2+ compounds), in an efficient and/or commercially viable manner.    DETAILED DESCRIPTION - Oxidative coupling of methane to generate hydrocarbon compounds containing at least two carbon atoms (C2+ compounds), involves: (a) mixing a first gas stream comprising methane with a second gas stream comprising oxygen to form a third gas stream comprising methane and oxygen, where (i) the temperature variation of the third gas stream is less than 10 degrees C, (ii) the variation of the concentration of methane to the concentration of oxygen (CH4/O2) in the third gas stream is less than about 10%, and/or (iii) the variation of the flow rate of the third gas stream is less than about 5%; and (b) performing an oxidative coupling of methane (OCM) reaction using the third gas stream to produce a product stream comprising one or more C2+ compounds. INDEPENDENT CLAIMS are included for:    (1) a reactor system for performing oxidative coupling of methane to generate hydrocarbon compounds containing at least two carbon atoms (C2+ compounds), which comprises: (a) a mixer comprising (i) a conduit providing a fluid flow path for a first gas stream comprising methane and (ii) a set of airfoil-shaped manifolds distributed across the fluid flow path, where the airfoil-shaped manifolds inject a second gas stream comprising oxygen into the fluid flow path to provide a third gas stream comprising methane and oxygen; and (b) a catalyst that performs an OCM reaction using the third gas stream to produce a product stream comprising one or more C2+ compounds;    (2) a method for producing at least one C2+ alkene, which involves: (a) directing methane and an oxidizing agent into a reactor comprising a catalyst unit and a cracking unit downstream of the catalyst unit, where the catalyst unit comprises an OCM catalyst that facilitates an OCM reaction, and where the cracking unit generates C2+ alkene from C2+ alkane; (b) reacting the methane and oxidizing agent with the aid of the OCM catalyst to generate at least one OCM product comprising at least one C2+ compound; (c) providing at least one OCM product in a hydrocarbon-containing stream that is directed through the cracking unit, which hydrocarbon-containing stream comprises at least one C2+ alkane; and (d) in the cracking unit, cracking the at least one C2+ alkane to provide the at least one C2+ alkene in a product stream that is directed out of the reactor, where the cracking unit is operated at a (i) hydrocarbon-containing stream residence time and (ii) cracking unit temperature profile selected such that the ratio of C2+ alkene to C2+ alkane in the product stream is greater than 0.1;    (3) an OCM system, which comprises: (a) a catalyst unit comprising an OCM catalyst that facilitates the reaction of methane and an oxidizing agent in an OCM reaction to generate an OCM product comprising at least one C2+ compound; (b) a cracking unit downstream of the catalyst unit, where the cracking unit accepts the OCM product in a hydrocarbon-containing stream that is directed from an inlet to an outlet of the cracking unit, and where the cracking unit generates at least one C2+ alkene from at least one C2+ alkane in the hydrocarbon-containing stream; and (c) one or more inputs to the cracking unit, where a given input among the one or more inputs is in fluid communication with the hydrocarbon-containing stream, and where the given input provides one or more C2+ alkanes to the cracking unit; and    (4) a method, which involves: (a) providing a reactor comprising (i) an OCM section comprising an OCM catalyst that facilitates the formation of OCM products from methane and an oxidizing agent, and (ii) a post-bed cracking section located downstream of the OCM catalyst section that facilitates cracking of at least a portion of the OCM products, where the OCM section and the post-bed cracking section are integrated in the reactor; (b) directing the methane and the oxidizing agent to the OCM section; (c) conducting OCM in the OCM section to generate the OCM products; (d) mixing the OCM products with a CO2 stream to produce a cracking stream; and (e) cracking at least a portion of the OCM products in the cracking stream in the post-bed cracking section.