A waste heat boiler and heat exchange process for recovering waste heat from a stream of heated gas is disclosed. The system includes a convection heat transfer chamber, a boiler tank, and a plurality of heat pipes thermally Interconnecting the convection heat transfer chamber with the boiler tank. Each of the heat pipes includes an evaporator section which is disposed in heat transfer relation with a stream of heated gas flowing through the convection heat transfer chamber, and a condenser section disposed in heat transfer relation with a volume of water disposed within the boiler tank. The evaporator sections and condenser sections are totally enclosed within the convection heat transfer chamber and boiler tank, respectively, and are connected in dosed cycle fluid communication with each other. The heat pipes contain a working fluid which is characterized by a thermodynamic cycle in which the working fluid is vaporized in the evaporator section and flows to the condenser section where it is condensed to the liquid phase and returns in a closed cycle to the evaporator section, thereby transferring heat energy from the heated gas stream in the convection heat transfer chamber to the volume of water contained in the boiler tank. In a regenerative arrangement, exhaust gas discharged from the convection heat transfer chamber is recycled to the input of the convection heat transfer chamber to provide high mass flow at low velocity for optimum efficiency. In another arrangement, superheated steam is provided by a pair of heat pipe boilers whose convection heat transfer chambers are connected in series, with the evaporator of the steam generating unit being located downstream of the superheat unit and the input of the superheat unit being the steam output of the steam generating unit.