Side-reboilers are often required for effective heat integration of distillation columns with the remainder of the plant. Side-reboilers reduce the consumption of expensive steam or hot oil in the main reboiler. This article examines how calculations should be performed in the vicinity of a side-reboiler. A rigorous analysis is made and compared with alternative approaches including that commonly employed in computer simulation. To demonstrate the differences a case study has been prepared by adapting a proprietary steady-state process simulator, and to aid understanding all approaches are illustrated as a McCabe-Thiele diagram. A simple economic evaluation shows that the more rigorous modeling exposes savings, which were previously hidden.