NOVELTY - The method comprises removing a partial volume flow from a methane-containing gas flow, treating the partial volume flow with an electrically generated plasma to produce a modified gas composition that has a methane content, which is lower than the methane-containing gas flow, adding a supplementary gas to the modified gas composition, selectively separating hydrogen, carbon monoxide, ethylene, propene and/or propane from the modified gas composition, and sending back the modified gas composition in the methane-containing gas flow. The methane-containing gas is a natural gas.    USE - The method is useful for modifying a volume flow of methane-containing gas in the treatment of fermentation gas.    ADVANTAGE - The method allows simple, economic and energy efficient modification of the volume flow of methane-containing gas in an environmentally friendly manner thus ensuring effective treatment and long term storage of the fermentation gas and enhancing the quality of the fermentation gas.    DETAILED DESCRIPTION - The method comprises removing a partial volume flow from a methane-containing gas flow, treating the partial volume flow with an electrically generated plasma to produce a modified gas composition that has a methane content, which is lower than the methane-containing gas flow, adding a supplementary gas to the modified gas composition, selectively separating hydrogen, carbon monoxide, ethylene, propene and/or propane from the modified gas composition, and sending back the modified gas composition in the methane-containing gas flow. The methane-containing gas is a natural gas or a fermentation gas. A Wobbe-index of the modified gas composition is measured continuously or intermittently, where a quantity of added supplementary gas is regulated with the measured value. A ratio of the Wobbe-index of the modified gas composition to the Wobbe-index of the methane-containing gas flow is 0.85:1 to 1:0.85. The plasma is produced by a power generated from renewable energy, by a dielectric barrier discharge in the partial volume flow or by an arc discharge in a hydrogen stream, which is produced by electrolysis of water or by selective separation of hydrogen from the modified gas composition.