This paper presents a methodology to integrate life cycle assessment (LCA) in models used to design energy conversion systems. It is illustrated by an application to a thermo-economic model for the multi-objective optimization of synthetic natural gas (SNG) production from woody biomass. The life cycle inventory (LCI) is written as a function of the parameters of the thermo-economic model. The obtained environmental indicators from the Life Cycle Impact Assessment (LCIA) are thus adapted to process design and scale. The conceived thermo-environomic model allows for taking into account the environmental impacts as a criterion in addition to economic and thermodynamic criteria in the process design and optimization.