This paper discusses strategies for the environomic optimization of renewable energy conversion technologies that are at the conceptual process design stage and produce multiple energy services, using Life Cycle Assessment (LCA). It is illustrated by an application to the thermo-chemical production of Synthetic Natural Gas (SNG) from lignocellulosic biomass, producing both SNG and electricity. The MJ of wood at the inlet of the process is selected as the functional unit. In a first time, the effects of process scale on environmental impacts are investigated using three different impact assessment methods. This is done by performing multi-objective optimization with the SNG production costs and the cumulated environmental impacts of each impact assessment method as the two objectives. The process size is included in the decision variables. The identified optimal size range varies depending on the impact assessment method. For all methods, the impacts increase with the process size in this optimal range. This is due to a joint effect of the biomass logistics and of the scaling of the gasifier, which leads to an increased resource consumption per unit of volume with an increasing size. In a second time, multi-objective optimization is conducted at fixed process size, using three objectives. The two first objectives are the SNG output and the electricity output, and the third one is either one of the three environmental indicators or the SNG production costs. Results show that the choice of the impact assessment method and of the hypothesis for electricity substitution have an important influence on the results, and favor either the production of SNG or of electricity. In all cases, process efficiency is one of the most important aspects for impact reduction.