Energy, economic and environmental modeling/analysis were performed to optimize a gas engine CCHP system using Genetic Algorithm (GA) optimization method for residential buildings. Optimization was performed for one and two gas engines for providing specified values of electricity, heating and cooling capacities which were varying during a year. An objective function named Relative Equivalent Uniform Annual Benefit (REUAB) was introduced. The goal, was finding system design parameters such as nominal power of gas engine(s), their partial load during a year, boiler heating capacity and the ratio of electrical cooling capacity (provided by electrical chiller) to the total cooling capacity (provided by both electrical and absorption chillers), to make the objective function (annual system profit) maximized. Results showed a bigger REUAB for one gas engine (284 x 10(3) $/year) in comparison with selecting two gas engines (264.7 x 10(3) $/year). Results of sensitivity analysis for one gas engine CCHP system with increasing equipment and unit energy costs was also reported. (C) 2016 Elsevier B.V. All rights reserved.