A system for cooling high-temperature gasifiers and method for its operation wherein cooling conduits extend vertically through the walls of the gasifier and are connected at their ends to a closed-water circulation system which incorporates heat exchangers for removing heat from the system. Boiling in the system is prevented, and good heat transfer characteristics are achieved, by maintaining a high pressure system, at least 40 bar, coupled with a flow velocity of between 5 and 7 meters per second at a maximum internal diameter of the cooling conduits of 51 millimeters. Means are provided for maintaining the temperature of the water exiting from the cooling conduits at least 10.degree. C below the boiling point at the pressure and flow rate of the system.The cooling tubes in the walls of the gasifier are covered by a ramming compound of refractory material held in place by expanded metal or mesh which curves around each tube but is spaced therefrom and welded to webs interconnecting the tubes in a gas-tight wall. The expanded metal supports the ramming compound but at the same time does not come into direct contact with the tube walls and thus eliminates points of maximum thermal loading where boiling might occur.