Isothermal vertical reactor (1) for a chemical reaction, comprising a shell (2) with internal gas-permeable walls (16, 18) defining an inlet distribution chamber (17) and a product collecting chamber (19), an open-top catalytic bed between said gas-permeable walls, and a tube heat exchanger (5) embedded in said catalytic bed, the tube heat exchanger (5) comprising a bundle of U-tubes, where an inlet (14) is disposed to direct a fresh charge in an upper chamber (21) over the catalytic bed and the U-shaped top portion of the tube bundle; a portion (F1) of the fresh charge passes through the catalytic bed with a mixed axial-transversal flow, and another portion (F2) of the fresh charge enters the inlet gas-distribution chamber (17) and passes through the catalytic bed with a substantially transversal flow.