The energy demand is increasing as well as the GHG emissions and the fossil fuels depletion. Public awareness on this topic significantly raised up in last decades. Nowadays conversion of residual biomasses in energy represents a real opportunity. However, the sustainability of such fuels has to be proven: Life Cycle Assessment is usually used for that purpose. Because of the increasing natural gas demand, scientists are searching for new gasification techniques and among them the hydrothermal gasification is extremely promising. In this chapter, the LCA of such a technology is presented. Contributions of land use change is highlighted. The chapter evaluates and compares effects of different allocation methods of raw material and it points out a rigorous approach for economic allocation. Furthermore, a section is devoted to the end of the plantation life and its consequence on carbon stock and GHG emissions. When the process is correctly integrated 1 MWh of heat and 16.06 kg of equivalent fertilizer are got by hydrothermal way with a significant reduction of impacts. Moreover, the climate change impact is cut down up to 91% and the fossil depletion decreases up to 96%. The study underlines the great dependency of impact reduction to allocation methods: thus, reduction percentages are 80% GHG emissions and 85% fossil depletion compared to the reference system.