It is the aim of the present invention to provide a system and a process for hydrothermal SNG production from waste biomass having a design of the product separation that not only considers the grid quality specifications for SNG, but also the recovery of the exergy potential of the crude and the supply of required heat for the plant. Processes and systems for hydrothermal SNG production from carbon- containing substrate are disclosed, comprising in general: a) generating a slurry having a predetermined solids content as calculated in dry mass; b) heating and pressuring the biomass slurry above 200°C and 50 bar; g)gasifying the hydrolysed biomass slurry,from which precipitated solids are optionally separated before and/or after the gasification, c) performing LV separation and gas separation at high pressure to generate a liquid stream and a vapour stream; d)for the liquid stream, liquid expanding with or without power recovery or evaporating and turbine expanding followed by LV separation; and e)valve expanding or reheating and turbine expanding of the vapour stream followed by combustion and expansion in a turbine, and/or electrochemical conversion to heat and/or power of the so-generated SNG gas and/or supply of the so- generated SNG gas to a gas grid.