The hot effluent gas stream comprising H.sub.2 +CO and entrained molten slag and ash from the partial oxidation of an ash-containing heavy liquid hydrocarbonaceous fuel, an ash-containing solid carbonaceous fuel, or mixtures thereof is passed in succession through a radiant cooling zone, a metal gas transfer line with internal metal heat transfer surfaces, and a convection cooling zone. By maintaining the internal metal heat transfer surfaces of the gas transfer line where turbulent flow of gas stream occurs at a temperature in the range of about 150.degree. F. to below 700.degree. F. by noncontact heat exchange with a coolant, substantially no molten slag or ash sticks to the metal heat transfer surfaces on the inside of the gas transfer line. Simultaneously, where laminar flow fo the gas stream within the gas transfer line occurs, slag and ash entrained in the gas stream is prevented from sticking to the inside metal heat transfer surfaces of the gas transfer line by maintaining the temperature of said inside surfaces in the range of about 700.degree. F. to 1,200.degree. F. by noncontact heat exchange with a coolant.