A coalescence model for ammonia cluster-cluster collisions is developed using molecular dynamics trajectory calculations. The MD calculations show that the sticking probability increases as the cluster size increases or the cluster temperature decreases. In addition, the sensitivity of the sticking probability to cluster size decreases as the temperature decreases. In the coalescence model, the Ashgriz-Poo semi-empirical formula is used for collisions between clusters larger than 100 and MD simulation results are used for smaller-cluster collisions. DSMC simulations are then performed on ammonia condensation jets with different stagnation pressures. By including the proposed coalescence model, the average cluster sizes and size distribution agree reasonably well with experimental data.