The emission of H2S gas is one of the main reasons for workers' death. The study area is a network of the Al-Hur area in Karbala city-Iraq. To assess the emission quantity of hydrogen sulfide, TOXCHEM program software was adopted to simulate three different diameters 630 mm, 315 mm and 1000 mm, the length of these pipes was 1000 m for each diameter. The methodology was built by collecting samples for the selected pipe diameters and the other necessary data to be included in the proposed model for a whole year. This model was calibrated within spring, and the model was validated for the other seasons. Model results are very close to the actual case by applying a statistical method, where the magnitudes of (R2) and RMSE close to 1.0 and 0.0 respectively. In addition, sensitivity analysis was studied with different levels of discharge, pH, temperature, slope, and roughness coefficient. Overall results showed that all emissions in these networks give us a high warning particularly, in summer. The study contributed to knowing the fatal rate and emission of hydrogen sulfide gas in the networks by using a TOXCHEM model. Another conclusion is to reduce hydrogen sulfide emissions in sewer networks by raising the value of pH.