NOVELTY - Producing biomethane, comprises e.g. converting carbon dioxide, carbon monoxide and hydrogen into methane and water using a methanation unit, splitting the obtained water into hydrogen and oxygen by electric current using an electrolysis device, supplying obtained hydrogen at least partially to the methanation unit, supplying carbon dioxide-containing biogas at least partially to the methanation unit, and supplying synthesis gas at least partially to the methanation unit.    USE - The method and the device are useful for producing biomethane (all claimed).    ADVANTAGE - The method and the device produce biomethane in cost-efficient manner.    DETAILED DESCRIPTION - Producing biomethane, comprises converting carbon dioxide, carbon monoxide and hydrogen into methane and water using a methanation unit, splitting the obtained water into hydrogen and oxygen by electric current using an electrolysis device, supplying obtained hydrogen at least partially to the methanation unit, producing methane and carbon dioxide-containing biogas by fermenting fermentable materials in at least one fermenter of a biogas plant, supplying obtained carbon dioxide-containing biogas at least partially to the methanation unit, producing a synthesis gas containing carbon monoxide, hydrogen, carbon dioxide and methane by thermally gasifying thermal gasifiable substances using a gasification- and/or oxidizing agent in at least one reactor of a gasification device, supplying obtained synthesis gas at least partially to the methanation unit, and supplying biomethane formed in the methanation unit to a recovery- and/or storage device, preferably a gas storage and/or a gas network. An INDEPENDENT CLAIM is also included for a plant for producing biomethane, comprising the methanation unit (4), the electrolysis device (14), the biogas plant (7), the gasification device (2), and at least one biomethane supply line guided to recovery- and/or storage device, preferably gas storage and/or gas network (18).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the plant for producing biomethane (Drawing includes non-English language text).    Gasification device (2)    Methanation unit (4)    Biogas plant (7)    Electrolysis device (14)    Gas network (18)