NOVELTY - The method involves recovering heat in flue gas stream downstream of a consumer unit such as a combustor (2) and using such heat to produce pressurized superheated steam. The steam pressurizes a vessel (8') included in a main receptacle unit (8), and is passed through the vessel to contact and heat-treat the biomass raw material loaded in the vessel, producing fuel gases. The fuel gases are supplied together with steam to the consumer unit under maintained pressure. Solid mass residues are intermittently discharged from the vessel.    USE - Method for producing energy.    ADVANTAGE - Production of fuel gas from solid fuels in a power plant is integrated to an extent utilizing a less complicated and more efficient process, resulting in improved plant costs and efficiency and total economy of the plant. The pressurized superheated steam is passed through the biomass raw material-loaded vessel, effectively contacting and heat-treating the raw material as well as pressurizing the steam to high temperature in efficient, energy saving manner. The steam also serves as an excellent protective gas, preventing fuel gas from unintentionally igniting, and serves as an energy carrier which allows for excellent and cost-effective energy recovery. The steam raises and maintains pressure in the main receptacle unit without additional measures. Intermittent operation of the vessel enables production of fuel gas having excellent properties. Continuous steam flow through the main receptacle unit also ensures efficient and fast heating of fuel as well as associated heat transport, preventing condensation of sticky substances on equipment. Since the biomass raw material used contains fibers, handling and treatment can be simplified using simpler gasifier equipment.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a plant for producing energy.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of a plant.    Combustor (2)    Turbine unit (3)    Steam generator (6)    Main receptacle unit (8)    Vessel (8')