A modification to the fully thermally coupled distillation system, or Petlyuk column, in which a postfractionator is added has recently been considered. The modified structure incorporates the postfractionator aiming for a better match in the composition of the interlinking stages of the Petlyuk column. In this paper we present a design method for the modified thermally coupled system with postfractionator. A shortcut method is used to provide the minimum reflux conditions of the system, and the boundary value method is used to complete the design task. The effectiveness of the design method is shown through the application to several case studies. The effect of the postfractionator on the performance of the Petlyuk system is also analyzed.