Biomass is a renewable, yet scarce resource since land is limited. This thesis addresses the question how to efficiently convert the available lignocellulosic biomass and biomass wastes to fuel and other useful energy services. In particular, it presents a systematic methodology for the conceptual design of thermochemical processes and demonstrates it at the production of Synthetic Natural Gas (SNG) through conventional biomass gasification and methanation, or hydrothermal gasification of biomass wastes. Through an appropriate mathematical decomposition of the design problem, thermo-economic process modelling, advanced process integration techniques and multi-objective optimisation are combined to provide a set of parameter- and scale-independent flowsheets for the optimal trade-off between several design targets. The results of various design studies consistently demonstrate that process integration plays a critical role in the synthesis of energy- and cost-efficient processes. It allows both for a rational energy recovery by cogeneration and process intensification. Considerable potential is furthermore assessed for combining several complementary processes for an appropriate and complete conversion of the resource, which might decidedly improve the environmental performance of fuel production from biomass and stresses the importance of a systematic process design.