NOVELTY - A heat exchanger (10) comprises first array of fluid conduits provided within a housing and configured to carry a first fluid; and second array of fluid conduits provided within the housing and is configured to carry the first fluid.    USE - The heat exchanger is used for producing hydrogen by feeding fuel into a reactor comprising housing having an inlet and an outlet, and a flow path extending within the housing from the inlet to the outlet, where the flow path comprises a convectively-heated catalytic steam reformer and a convectively-cooled water gas shift reactor, where hydrogen is produced. A burner is provided to heat the steam reform. The combustor product from the burner is modulated by a flow diverting mechanism to maintain a predetermined operating temperature of the steam reformer. The combustion air is used to cool the water gas shift reactor before the combustion air is provided to the burner. The combustion gas is preheated before being used to cool the water gas shift reactor. It is also preheated to a temperature where the combustion gas is provided to the burner at a predetermined temperature that is not greater than an allowable burner inlet temperature. (all claimed)    ADVANTAGE - The invented heat exchanger facilitates accurate dynamic control of the process temperatures during all operating modes.    DETAILED DESCRIPTION - A heat exchanger comprises housing; first array of fluid conduits provided within the housing and configured to carry a first fluid; second array of fluid conduits provided within the housing and is configured to carry the first fluid; and first fluid passageway provided within the housing. The first fluid passageway is defined by an internal surface of the housing and by a baffle plate. It is configured to carry a second fluid. The baffle plate is configured to divide the first fluid passageway into a first flow path (82) and a second flow path (84). The first array of fluid conduits extends through the first flow path and the second array of fluid conduits extends through the second flow path.    DESCRIPTION OF DRAWING(S) - The figure is a front, cross-sectional view of a heat exchanger.    Heat exchanger (10)    Baffle plates (34, 36)    Gap (37)    Sealing zone (70)    Flow path (82, 84)