Compact reaction apparatus, such as for steam reforming a hydrocarbon feedstock to produce hydrogen, includes a plurality of reactor tubes disposed within a furnace. A portion of each reactor extends into the burner cavity or combustion volume of the furnace. Baffles, such as sleeves, are disposed around these portions of the reactor tubes to shield the tubes from excessive radiant heat from the wall of the burner cavity and to more evenly distribute heat among and around all of the reactors. These baffles permit the reactor tubes to be closely packed within the furnace and reduce temperature differences between the tubes.