Abstract: In a reactor comprising an outer shell and an internal cartridge preferably obtained by assembling a number of stackable modular cartridges, each comprising from the outside to the inside at least one solid wall forming an air space with the shell inside wall, a continuous bottom to the solid wall, a first wall perforated for the whole of its length, a second internal wall only partially perforated, and a catalytic bed inserted between the said bottom and said walls totally respectively partially perforated, at least one heat exchanger is arranged centrally and axially in a catalytic bed; the feed gas which has cooled the external wall of the shell collects in a duct in the middle of the exchanger along which it runs in one way (for example, from top to bottom) running successively along the exchanger in the other way inside the exchanger tubes emerging preheated in the free zone over the top layer of the catalytic bed where it is mixed with fresh gas fed directly from the outside of said zone.