This paper presents a methodology to integrate life cycle assessment (LCA)in thermo-economic models used for the optimal conceptual design of energy conversion systems. It is illustrated by an application to a thermo-economic model developed for the multi-objective optimization of combined synthetic natural gas (SNG) and electricity production from lignocellulosic biomass. The life cycle inventory (LCI) is written as a function of the parameters of the thermo-economic model. In this way, the obtained environmental indicators from the life cycle impact assessment (LCIA) are calculated as a function of the decision variables of process design. The LCIA results obtained with the developed methodology are compared with the results obtained by a conventional LCA of the same process. Then, a multi-objective environomic (i.e. thermodynamic, economic, environmental) optimization of the process superstructure is performed. The results highlight the important effects of process configuration, integration, efficiency and scale on the environmental impacts.