Abstract: An exothermic heterogeneous catalytic synthesis reactor including at least one catalytic bed 5a-5c arranged in a cylindrical shell 2 and provided with a bottom plate 6a-6c for containment of the catalyst, and at least one supporting shoulder 7a-7c for the bottom plate 6a-6c extending from the shell 2, includes a catalyst-seal support device including an annular element 17, 25 placed between the shoulder 7a-7c and the bottom plate 6a-6c. The annular element 17, 25 is fixed in a removable manner with the bottom plate 6a-6c and has a thermal expansion coefficient substantially equal to that of the shell 2.