Based on a previously developed thermo-economic process model, this paper presents a detailed design study for the polygeneration of Synthetic Natural Gas (SNG), power and heat by catalytic hydrothermal gasification of biomass and biomass wastes in supercritical water. Using multi-objective optimisation techniques, the thermodynamic and thermo-economic performances of all candidate configurations from a general process superstructure are optimised with respect to SNG and electricity cogeneration and its associated investment cost, production cost and plant profitability. The paper demonstrates how both the optimal system configuration, its operating conditions and performances depend on the available technology, catalyst lifetime, process scale and the characteristics of the processed substrate.