Sensible heat in the hot effluent gas stream leaving a partial oxidation gas generator for the production of gaseous mixtures comprising H.sub.2 +CO i.e. synthesis gas, reducing gas, or fuel gas, is used at maximum temperature to heat a stream of heat transfer fluid preferably comprising a portion of the product gas circulating in a substantially closed loop. The heat transfer fluid serves as the working fluid in a turbine that produces mechanical work, electrical energy, or both. Further, the heat transfer fluid leaving the turbine may be used to preheat hydrocarbonaceous feed and free-oxygen containing gas which is then introduced into the gas generator. Optionally, by-product superheated steam may be produced at high temperature levels for use in the gas generator and as the working fluid in an expansion turbine. The high steam superheat temperatures results in higher conversion efficiencies.