The invention relates to a process and to an apparatus for effecting chemical syntheses in gaseous phase, under pressure, in the presence of a solid catalyst, for instance, for ammonia synthesis from hydrogen and nitrogen or for the synthesis of methanol or higher homolog alcohols from hydrogen and at least one carbon oxide.The reactor (8) of substantially cylindrical shape, (see FIG. 4) contains a plurality of elongate compartments (9) of parallelipiped shape, adjacent to each other, the adjacent walls of the compartments or the common walls of the adjacent compartments being gas-tight walls (1), said tight walls forming hollow plates wherein are provided channels for the flow of a fluid heat carrier flowing through said walls under a pressure substantially equal to the pressure to which are subjected the reaction gases.