A methodology for the quantification of entropy generation and energy consumption in isothermal, isobaric reactor networks is presented. The proposed methodology employs the Infinite Dimensional State-space (IDEAS) conceptual framework, which is shown to be applicable to the problem under consideration. The IDEAS framework considers all possible reactor units, and all possible mixing and splitting interconnections among them. It will be shown mathematically that, under certain conditions, entropy generation and energy consumption are functions of only the inlet and outlet stream compositions and flow rates and are not dependent on the reactor network structure, as long as there exists a network able to deliver the considered outlets from the known inlets. This theoretical result provides the foundation for a graphical method that can quantify entropy generation and energy consumption, by first identifying the reactor network's Attainable Region (AR) and then depicting the behavior of the entropy generation and energy consumption functions within the AR. The proposed methodology is demonstrated on a case study featuring reversible reactions both in series and in parallel. Finally, conclusions are drawn.