NOVELTY - The heat exchanger comprises a housing (1, 6, 7) and a first heat exchanger stage (3) and a second heat exchanger stage (5), which are arranged in the housing. Hot gas stream is subjected by a heat flowing through the housing in primary side. The first heat exchanger stage is arranged for evaporating water and the second heat exchanger is arranged downstream to the first heat exchanger for heating steam produced in the first heat exchanger stage. The first and the second heat exchanger stages are formed as tube heat exchanger.    USE - Heat exchanger for processing process gas in a fuel cell plant with internal and/or external fuel gas reforming (claimed).    ADVANTAGE - The heat exchanger can be simply manufactured with good heat exchanging capability.    DETAILED DESCRIPTION - The heat exchanger comprises a housing (1, 6, 7) and a first heat exchanger stage (3) and a second heat exchanger stage (5), which are arranged in the housing. Hot gas stream is subjected by a heat flowing through the housing in primary side. The first heat exchanger stage is arranged for evaporating water and the second heat exchanger is arranged downstream to the first heat exchanger for heating steam produced in the first heat exchanger stage. The first and the second heat exchanger stages are formed as tube heat exchanger and arranged with vertically directed axis in the housing. An inlet for mixing hydrocarbon containing fuel gas to be reformed to the water steam led by the first heat exchanger stage to the second heat exchanger stage. The first heat exchanger stage and the second heat exchanger stage are arranged one above the other in the housing flowing by the hot gas stream. The heat exchanger stages are arranged in a ring shaped column between the housing and an extrusion body (2) arranged within the housing. The first heat exchanger stage for evaporating the water is flow-through in the direct current to the gas guiding the heat. The heat exchanger stage for heating the steam is flow-through in the counter current to the gas guiding the heat. The housing is flow-through by the gas guiding the heat from gas inlet arranged in the bottom from bottom towards top to gas outlet arranged in top. The second heat exchanger stage for super heating the steam is arranged over the first heat exchanger stage. A steam separator (4) for separating the steam to be brought to the second heat exchanger stage is arranged between the steam outlet of the first heat exchanger stage and the steam outlet of the second heat exchanger stage by water delivered from the first heat exchanger stage and the water is formed according to the type of cyclone. The steam separator is arranged within the extrusion body, which is formed as steam separator. The first heat exchanger stage and the second heat exchanger stage are arranged one above the other in the housing flowing by the hot gas stream. The second heat exchanger stage is arranged over the first heat exchanger stage and the housing is flow-through from the gas inlet to the gas outlet. The coiled tubes of the heat exchanger stages are arranged in a ring-shaped column between the housing and the extrusion body. The steam separator is arranged in the interior of the extrusion body, which is formed as steam separator.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of a heat exchanger for processing process gas.    Housing (1, 6, 7)    Extrusion body (2)    First heat exchanger stage (3)    Steam separator (4)    Second heat exchanger stage. (5)