In this study, first and second laws of thermodynamics, accompanying with Rational Exergy Management Method (REMM) were employed in developing a MATLAB based algorithm for natural gas fired, internal combustion engine (ICE) driven poly-generation systems. Two systems were studied based on a tri-generation plant built within the framework of the EU-FP6 HEGEL Project, tested at METU MATPUM (RICBED) building. This study introduces a better set of metrics for rating, evaluating, and optimizing poly-generation systems in order to minimize emissions, maximize fuel savings, and thus to accomplish an optimum sustainability metric among the factors of environment, energy, human needs and economics. Results show that with ICE poly-generation systems, exergy efficiency may increase beyond 60%. Even at part loads, minimum values of Primary energy savings (PES) are 12.4% for Case-1 and 17.7% for Case-2 (compared to minimum allowed 10%). REMM efficiency and Exergy Embedded PES (PES(R)) evaluated by REMM are proven to be better indicators of the performance. When exergy destruction is lower, (waste heat is recovered) PES(R) increases significantly. PES(R) values are minimum 18.2% for Case-1 and 42.4% for Case-2, which reveals that both systems provide high performance energy generation and considerably lower emissions. (C) 2011 Elsevier B.V. All rights reserved.