A gas/liquid, parallel flow heat exchanger in a waste-heat tank which is constructed as a pressure tank, is charged with coolant, and is connected after a pressure gasification. The waste-heat tank includes conduit-like elements of equal length which serve as heat transfer surfaces and are installed in conduit-like components or wall units. The wall units divide the interior of the waste-heat tank into regions having upwardly and downwardly directed fluid flow. A central, coaxial, polygonal, preferably hexagonal, cylindrical displacement body is provided, with the number of surfaces of the displacement body depending upon the number of heat transfer surface elements. Also provided are wall units which project out of the wall of the container of the waste-heat tank into the interior thereof across from the corners of the polygonal displacement body. Cylindrical displacement bodies are located centrally between the corners of the polygonal displacement body on an imaginary circle. The heat transfer surface elements, in turn, are coaxially arranged about these cylindrical displacement bodies.