a system and method for producing product gas using residual waste liquor is described with a gasifier reactor having a fluidized bed located therein . the gasifier reactor is heated to a predetermined temperature range with either an external heater or a second fluidized bed located at a position below the first fluidized bed . a heat exchanger may be positioned in the first fluidized bed and / or the second fluidized bed for indirectly heating the respective fluidized beds . condensing heat exchanger means recovers heat from the product gas and condenses an acid gas therefrom for recycling the chemicals . a reagent is sprayed in the condensing heat exchanger means to clean the product gas . pressurization allows the cleaned product gas to be directly fired in a turbine .