In this work a MINLP formulation for the optimal design of 1-2 shell-and-tube heat exchangers in series is proposed. The optimization model consists of the set of equations based on the X-P design method and inequalities constraints that ensure feasible and practical heat exchangers. The goal is to choose the design parameters of the multipass heat exchangers in series that minimizes the investment cost. The results indicate that optimal designs of multipass heat exchangers can be automatically determined with the proposed design strategy.