The invention concerns an exchanger-reactor (1) comprising: a vessel (2); means for distributing a feed through a fixed bed catalytic zone (10); means (6) for collecting effluent from the catalytic zone (10); means for heating the catalytic zone (10); in which said collection means (6) comprise conduits passing right through the catalytic zone (10), said conduits being distributed in the catalytic zone and interposed between the heating means, and in which the heating means of the catalytic zone are contained in sheaths (8) which are partially immersed in the catalytic zone (10), the sheaths (8) being open at one of their ends and closed at the other, the open end being fixed to an upper tube plate (21) defining the collection chamber (19) which is located above the catalytic zone (10), said heating means comprising at least one combustion zone (13) located close to the catalytic zone, means for supplying said combustion zone (13) with an oxidizing gas mixture (15) and with a gaseous fuel (17), and means for evacuating the gaseous effluent deriving from the combustion (14).