TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The carbon dioxide separated from the gas stream is supplied to the second treatment site, and the supplied carbon dioxide forms at least one portion, preferably the total carbon content of the carbon dioxide of the reactant gas. The hydrogen is at least partially separated from the gas stream and supplied to the reactant gas. The reactant gas is supplied to an existing base gas containing carbon dioxide and methane for forming the gas stream. The base gas is a gas obtained from a biogas plant, preferably an existing desulfurized crude biogas that has only a minimum amount of atmospheric nitrogen and oxygen due to the formation process. The base gas is taken from a gas network (50), preferably an L-gas network, which is under predetermined feed conditions. The reactant gas at least partially, preferably completely comprises base gas containing carbon dioxide and methane, preferably crude biogas, to which hydrogen is added. The method comprises conditioning the supplied gas with the very high methane content according to predetermined feed conditions for supplying into the gas network, subjecting to the feed conditions, and supplying into the gas network. At least one major part of the hydrogen of the reactant gas is produced electrolytically, preferably adjacent to the second treatment site. The addition of the carbon dioxide-containing portion of the reactant gas is controlled depending on the electrical energy provided for the electrolysis, preferably time variable. The methanization reduces the hydrogen content of the product gas and increases the carbon dioxide content. The separation of carbon dioxide from the gas stream and/or the separation of hydrogen comprise gas scrubbing, amine scrubbing and/or pressure swing adsorption method. Preferred Components: The methane content of the supplied gas is greater than or equal to 92 vol.%, preferably greater than or equal to 98 vol.%. The supplied gas and/or the product gas comprise (in vol.%) carbon dioxide ( less than or equal to 4, preferably less than or equal to 1), and hydrogen ( less than or equal to 4, preferably less than or equal to 1).    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The plant comprises a controlling device for controlling the plant. The line system exhibits a connection of an outlet of the separating device for the separated carbon dioxide and/or the separated hydrogen to a gas inlet of the methanation device, and a connection of a gas outlet of the biogas plant to a gas stream inlet of the separating device and/or to the reactant gas inlet of the methanation device.