A process for the heterogeneous synthesis of chemical compounds such as methanol and ammonia through catalytic conversion of the respective gaseous reactants that are made to pass through a first (2) and a second (3) reaction zone connected in series with each other, in which they react in pseudoisothermal conditions, distinguishes itself in that in the first reaction zone (2) the gaseous reactants are made to flow through a fixed mass of an appropriate catalyst in which a plurality of substantially box-like, plate-shaped heat exchangers (21), arranged side-by-side and crossed by a heat exchange operating fluid, is dipped.