Reactor for gasifying a carbonaceous fuel to produce a usable gas, and comprising an insulated outer shell. A reaction chamber within the shell receives a fuel mixture from an injection burner. The products or effluent of gasification include hot produced gases which are passed through a constricted throat to be cooled in a liquid bath. A dip tube, which guides the hot effluent into the bath, is provided with a stream of coolant from a quench ring. The latter is supported below the reaction chamber and is provided with a thermal barrier along its surface that is exposed to the hot produced gas. The barrier will reduce thermal stress in the quench ring and reduce drain of heat from the gasifier throat, thus avoiding premature chilling of the slag component.