The proposal concerns a stage in the process of cooling the synthetic gas produced in a coal gasification plant at temperatures above 900/sup 0/C. The purpose is to keep the convection heating surface of the subsequent waste heat plant free of dirt. According to the invention, the waste heat plant has a radiation area connected before it, on the heating surfaces of which the slack carried over solidifies. This radiation area has a hydraulic and thermal cleaning system, which can be raised or lowered in a water bath. The subclaims concern all the constructional characteristics of this cleaning system, which causes the solidified slack to crack.