The technical, economic, and ecological feasibility of the co-production of 50 PJ of Fischer-Tropsch (FT) transportation liquids per year and 150 PJ of synthetic natural gas (SNG) per year (i.e., 10% of the 2001 Dutch consumption) has been studied. (Note: PJ is the abbreviation for petajoule; hence, 1 PJ = 10(15) J.) In the co-production concept, some of the SNG is produced by methanation of the FT off-gas, which already contains significant amounts Of C-1-C-4 SNG compounds. The additional required SNG is produced by dedicated methanation of some of the gasification product gas. Co-production results in higher biomass-to-fuel efficiencies, lower biomass input requirements, and less-negative net present values (NPVs), compared to the case of complete separate production of both fuels. Co-production concepts that are based on pressurized oxygen-blown gasification result in the lowest CO2 emission reduction costs (similar to 100 Euro/tonne) and with a potential reduction to 55 Euro/tonne, considering CO2 sequestration. However, the CO2 emission reduction costs of atmospheric steam-blown indirect gasification are in the same range. Co-production of "green" FT transportation fuels and "green" SNG will be an economic feasible process in The Netherlands when both energy carriers receive the same tax exemptions as those currently given to green electricity.