Pseudo-isothermal chemical reactor (1) for catalytic reactions with a vertical axis, comprising a substantially cylindrical shell (2), Closed at the opposite ends by upper (4) and lower (3) bottoms respectively, a reaction zone (8) in which a catalytic bed (11) and a plurality of flat, boxed, plate-shaped heat exchangers (12), having the shape of a parallelepiped and having long sides parallel to said vertical axis and short sides perpendicular to it, situated in said reaction zone and supported in an arrangement on parallel cords at a predetermined distance.