NOVELTY - The method for operating a solid oxide fuel cell by a fuel containing heavy hydrocarbons, oxygenous gas and water vapor, comprises introducing the fuel into a first reactor (4) prior to its delivery into a solid oxide fuel cell (6), and introducing the fuel into a second reactor (5). In the first reactor the heavy hydrocarbons are degraded into carbon monoxide and hydrogen by partial oxidation and steam reforming of the hydrocarbons, where heat required for the reaction is released simultaneously. In the second reactor the produced hydrogen and carbon monoxide are reacted to form methane.    USE - The method is useful for operating a solid oxide fuel cell by a fuel containing heavy hydrocarbons, oxygenous gas and water vapor (claimed).    ADVANTAGE - The method is capable of efficiently operating a solid oxide fuel cell when electricity is generated at a relatively high efficiency and heat at such a temperature level that it can be utilized for instance for heating buildings or for producing steam.    DETAILED DESCRIPTION - The method for operating a solid oxide fuel cell by a fuel containing heavy hydrocarbons, oxygenous gas and water vapor, comprises introducing the fuel into a first reactor (4) prior to its delivery into a solid oxide fuel cell (6), and introducing the fuel into a second reactor (5). In the first reactor the heavy hydrocarbons are degraded into carbon monoxide and hydrogen by partial oxidation and steam reforming of the hydrocarbons, where heat required for the reaction is released simultaneously. In the second reactor the produced hydrogen and carbon monoxide are reacted to form methane. The fuel containing methane is fed into the solid oxide fuel cell. In the first reactor a temperature is brought to 600-700degrees C. In the first reactor the degradation of heavy hydrocarbons takes place in a vicinity of a catalyst containing noble metal. In the second reactor the compounding of hydrogen and carbon monoxide into methane takes place in a vicinity of a copper-based or a nickel-based catalyst. An INDEPENDENT CLAIM is included for a solid oxide fuel cell plant for energy production by utilizing a fuel containing heavy hydrocarbons.    DESCRIPTION OF DRAWING(S) - The diagram shows a simplified system for operating a solid oxide fuel cell.    Supply line (1)    First reactor (4)    Second reactor (5)    Solid oxide fuel cell (6)    Second feed conduit. (8)