Activated sludge produced from biological treatment of textile and leather industrial wastewater was processed using hydrothermal gasification in order to produce fuel gas with high calorific value. The gasification experiments were performed in a batch reactor and the effect of temperature (400, 450, 500 and 550 degrees C), additives (KOH and dolomite) and reaction time (0, 30 and 60 min) was investigated. The gas yield increased with the increasing temperature. Hydrogen and methane compositions in the gas was between 42 to 57 vol%, while CO2 was between 30 and 40 vol%. The calorific value of the sludge was determined as around 16 MJ/kg, while after gasification the calorific value of the gas fuel produced was found to be 24.7 MJ/kg (24.3 MJ/Nm(3)) after 30 min. reaction time in the presence of KOH. The addition of dolomite did not affect the gas yield however, addition of KOH promoted the water-gas shift reaction and boosted hydrogen yield in the product gas. Around 70 wt. % of the sludge was converted into either gas or liquid with hydrothermal treatment.