NOVELTY - The method comprises operating an electrolysis unit of a steam plant to provide hydrogen and oxygen, operating a combustion chamber (110) of the steam plant with a first fuel and the oxygen, which are supplied by the electrolysis unit, and operating a methanation unit (300) of the steam plant under consumption of hydrogen, which is supplied by the electrolysis unit and of carbon dioxide, which is provided by the combustion chamber. The steam plant comprises a waste heat steam generator for preparing steam using thermal energy and providing the carbon dioxide as an exhaust gas.    USE - The method is useful for operating a steam plant for producing electrical energy by a combustion process according to an oxyfuel method and a vapor process (claimed).    ADVANTAGE - The method allows efficient operation of the steam plant thus producing the electrical energy in an environmentally-friendly and economical manner.    DETAILED DESCRIPTION - The method comprises operating an electrolysis unit of a steam plant to provide hydrogen and oxygen, operating a combustion chamber (110) of the steam plant with a first fuel and the oxygen, which are supplied by the electrolysis unit, and operating a methanation unit (300) of the steam plant under consumption of hydrogen, which is supplied by the electrolysis unit and of carbon dioxide, which is provided by the combustion chamber. The steam plant comprises a waste heat steam generator for preparing steam using thermal energy and providing the carbon dioxide as an exhaust gas. The electrolysis unit is configured for decomposition of water into hydrogen and oxygen. The methanation unit is configured for synthesizing methane and/or methanol under the consumption of hydrogen and carbon dioxide. The hydrogen is provided from the electrolysis unit to a first gas reservoir. The oxygen is provided from the electrolysis unit to a second gas reservoir. An electrical energy is supplied to electrolysis unit using a regenerative energy source (210). The electrolysis unit is supplied by electrical energy from steam process of the steam plant. The steam plant further comprises a gas turbine (120) that is operated under the combustion of a gaseous second fuel. The exhaust gases are partially supplied for facilitating the production of steam. The gas turbine is operated under the consumption of hydrogen as the second fuel using the electrolysis unit. Heat is removed from the methanation unit during operation, and is supplied to the gas turbine for pre-heating the second fuel. The water is separated from the exhaust gases of the gas turbine using the electrolysis unit, and is stored in a water reservoir (220) before supplying to the electrolysis unit. The heat removed from the methanation unit is stored in a heat accumulator. The gas turbine includes compressor and turbine stages, which are mechanically separated from one another. A part of a compressed air is supplied to the turbine stage from the compressor stage. The compressor stage compresses the air by thermal interaction under thermally conditions using a heat source. The heat source is identical with the methanation unit for supplying heat to the heat accumulator. An INDEPENDENT CLAIM is included for a steam plant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a steam plant.    Combustion chamber (110)    Gas turbine (120)    Energy source (210)    Water reservoir (220)    Methanation unit. (300)