Heat exchange unit for pseudo-isothermal reactors including a substantially cylindrical shell (2) closed at its opposite ends by respective bottoms (3, 4), at least one thereof is provided with at least one manhole opening (5) of predetermined dimensions, a reaction zone (6) inside the shell (2) in order to contain a catalytic bed, comprising at least two modular and assembly heat exchangers (11), having predetermined cross dimensions smaller than those of the manhole opening (5), each heat exchanger (11) comprising at least one heat exchange element (12).