Operating method for a power plant

The method involves expanding hot gas from a higher pressure stage combustion chamber into a turbine whilst outputting mechanical power and feeding it to a lower pressure stage combustion chamber. Expanded hot gas flows through a waste heat vessel. Steam is produced and fed to a reactor with fuel containing hydrocarbons to produce a reactor gas. The reactor gas fed mainly to a combustion chamber in the highest possible pressure stage and burnt. The method involves expanding hot gas (63) from a higher pressure stage combustion chamber into a turbine (13) whilst outputting mechanical power and feeding it into a lower pressure stage combustion chamber, whereby an expanded hot gas (66) flows through a waste heat vessel (2) in which steam (52) is produced and fed to a reactor (22) with fuel (53) containing hydrocarbons to produce a reactor gas (55). The reactor gas fed at least predominantly into a combustion chamber in as high a pressure stage as possible and burnt.