This paper addresses a methodology for the optimal conceptual design of thermochemical fuel production processes from biomass. A decomposed modelling approach with separate energy-flow, energy-integration and economic models are coupled with a multi-objective optimisation strategy. It is applied to the design of a process that produces synthetic natural gas (SNG) from lignocellulosic materials. The systematic choice of the objectives thereby assures the generation of a general set of optimal process flowsheets, which constitute a sound basis for the synthesis of a viable plant. Statistical methods are used to realise a detailed multi-criteria analysis of the results.