NOVELTY - A catalyst is added to integral reactor, reducing gas of hydrogen and nitrogen are introduced into reactor through bottom of reactor, and reduction reaction is performed. Paraffinic hydrocarbon is introduced from top of reactor and methanation is initiated. The gas-phase product is discharged from top of reactor. A portion of gas-phase product is recycled to compressor. Another portion of gas-phase product is introduced into heat exchanger to cool, cooled product is introduced into gas-liquid separator (S1), and natural gas is discharged from top of reactor.    USE - Methanation using catalyst slurry bed (claimed).    ADVANTAGE - Methanation is performed using the catalyst in integral reactor, without separating the catalyst and with reduced energy consumption, by simple method.    DETAILED DESCRIPTION - A catalyst is added to an integral reactor, reducing gas of hydrogen and nitrogen are introduced into the reactor through the bottom of the reactor, reaction temperature is increased, and reduction reaction is performed under normal pressure. Paraffinic hydrocarbon is introduced from the top of the reactor, reducing gas is switched to synthesis gas, and methanation is initiated. The gas-phase product is discharged from the top of the reactor. A portion of gas-phase product is recycled to a compressor, and obtained synthesis gas is recycled into the reactor from the bottom of the reactor. Another portion of gas-phase product is introduced into heat exchanger to cool, cooled product is introduced into the gas-liquid separator (S1), liquid condensate is discharged from the bottom of the gas-liquid separator, and natural gas is discharged from the top of the reactor. The paraffinic hydrocarbon is flowed downward from the top of the reactor, and the catalyst layer and synthesis gas is contacted. The resultant gas product is introduced into a gas-liquid separator (S2), liquid paraffin is discharged from the bottom of the separator (S2) and gas-phase product is discharged from the top of the separator (S2). The liquid paraffin is cooled in heat exchanger, and recycled into the reactor using circulation pump. An INDEPENDENT CLAIM is included for integral reactor.