A horizontal, cold wall, multi-bed ammonia converter having two, shell and tube interchangers for cooling reaction gas streams leaving respectively the first and second catalyst beds with incoming reactant gases wherein each of the interchangers is vertically oriented and physically located between catalyst beds. The catalyst beds are adiabatic, slab-shape beds defined in part by side portions of the inner wall of the converter and are arranged for downward, transverse flow of reacting gases. Longitudinal conduits and baffling in cooperation with the arrangement of catalyst beds and interchangers provide serial flow of gas through the cold side of each of the interchangers, a first catalyst bed, the hot side of one of the interchangers, a second catalyst bed, the hot side of the other interchanger, a third catalyst bed, and, optionally, a fourth catalyst bed.