An improved continuous partial oxidation process for producing clean synthesis or fuel gas and by-product steam from a hydrocarbonaceous fuel feed is disclosed wherein hot gases from the process are simultaneously cooled and cleaned by direct contact with hydrocarbon oil. Thus, the effluent gas stream from the reaction zone of the gas generator is cooled and cleaned by discharging the gas stream directly into a relatively large body of hot liquid hydrocarbon immersion fluid. For example, the immersion fluid may be a dispersion of liquid hydrocarbonaceous fuel such as petroleum oil and particulate carbon which is contained in a separate immersion vessel. A portion of the hot immersion fluid is continuously removed from the immersion vessel and cooled to a temperature in the range of about 300.degree. to 850.degree. F. but above the dew point of the water in the process gas stream in an external heat exchanger, such as a steam generator. High pressure by-product steam may be produced thereby. A portion of the cooled immersion fluid, optionally in admixture with a hydrocarbonaceous scrubbing fluid of a similar composition obtained subsequently in the process may be recycled to the immersion vessel and optionally to the gas generator as at least a portion of the hydrocarbonaceous fuel feed.