A technique of cooling of a reactor with a fixed catalytic bed for exothermic catalytic syntheses, such as the Fischer-Tropsch (FT) synthesis of liquid fuels by selective hydrogenation of the monoxide of carbon (CO) by the hydrogen (H.sub.2), includes mixing of these two gases being called "synthesis gas" or "syngas", and is characterized by the fact that the heat of such synthesis is taken away by a heat medium of non-fossil origin and having a high boiling point in comparison with water and that at least a part of the resultant vapors of such boiling heat medium is extracted from the reactor to feed, as a carbonaceous matter, another reactor, called "reformer", in which this vapor is converted into syngas to feed the aforesaid reactor of exothermic synthesis.