The hot effluent gas stream comprising H.sub.2 +CO and entrained molten slag 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 or quench cooling zone, a metal gas transfer line and a convective cooling zone. By covering the metal surfaces of the gas transfer line or the gas transfer line and the convective cooler connected thereto with a high temperature resistant carbon-containing material, substantially no molten slag sticks to the metal heat transfer surfaces on the inside of the gas transfer line or to the surfaces of the gas transfer line and the convective cooler connected thereto which are contacted by the partially cooled gas stream leaving the radiant or quench cooling zones.