This study evaluated energy recovery from an upflow anaerobic sludge blanket (UASB) reactor treating paper industry wastewater and the goodness of fit of the Mantis2 model (i.e., using version 6.5 of the GPS-X (TM) software platform) to biogas and the effluent total chemical oxygen demand (COD) data. The UASB reactor achieved stable performance, exhibiting high COD removal efficiency about 96% for COD/SO42- = 5.0 on average. Experimental findings also revealed up to approximately 300 L/kg-CODrem. biogas yield indicating no clear inhibition due to high sulfate content. Daily biomethane production corresponded to approximately 25 kWh total energy (electricity+heat) generation per 1 m(3) of wastewater treated. The overall goodness of fit of the model was satisfactory, based on the relatively low absolute deviations of 6.0% and 12.7% from the measured data for effluent COD and biogas generation, respectively. Besides, the resulting effluent COD simulation yielded a coefficient of determination (R-2) value of 0.7663 and a mean absolute error (MAE) of about 39 mg/L. In light of experimental and simulation findings, the lab-scale UASB reactor indicated satisfactory performance and substantial biogas generation which would further encourage full-scale implementations of these promising energy recovery technologies in paper industries.