A method is provided for turning down from design conditions or below the output of raw product gas from a free-flow partial oxidation gas generator, or when operating below design conditions the output of raw product gas from the gas generator may be turned up while maintaining substantially constant the efficiency of the gas generation, or conversion of the fuel to gas, and the quality of the gas produced. The flow rates of the feedstreams to the burner are adjusted down or up a predetermined amount for each of the feedstreams to the burner to obtain a specified output of raw product gas while maintaining substantially constant in the reaction zone the levels of O/C atomic ratio and the H.sub.2 O/fuel weight ratio. Further, a pressure adjustment in the system is made which produces a corresponding adjustment to the pressure in the gas generator. The adjusted pressure is a direct function of the adjusted flow rate for the adjusted fuel or oxidant feedstream.