The invention comprises a gasification reactor which comprises a vessel having at its upper end a downwardly directed burner and a combustion chamber located in the upper half of the vessel. The wall of the combustion chamber comprises an arrangement of interconnected parallel arranged tubes running from a common lower arranged distributor to a higher arranged common header. The distributor is connected to a cooling water supply conduit and the header is connected to a steam discharge conduit. Both the steam discharge conduit and the water supply conduit are fluidly connected to a steam drum. The steam drum is provided with a supply conduit for fresh water and the steam drum is positioned at a higher elevation than the common header.