Synthesis gas is reacted in several catalytic beds having axial-radial or radial flow. Reacted gas is collected at an outlet of a final catalytic bed and is transferred to a reaction heat recovery system situated at a top of a reactor. The reactor includes three catalytic beds, two or more beds having inverted, curved bottoms. A first quenching system is located in the reactor and includes a distributor situated inside a first, upper bed at a location immediately under an unperforated portion of an internal wall of that bed. A gas/gas heat exchanger is located centrally within one or more of two upper beds located within the reactor. A water pre-heater or boiler is located inside an upper bottleneck portion of a shell of the reactor and is fed with reacted gas collected from a lowermost catalytic bed.