Numerical modelers are often faced with the challenge to numerically reproduce the physical behavior of soil-geosynthetic interaction problems using assumed values for missing soil and interface model parameters. This paper examines two such examples using the finite-difference method, (1) horizontal pullout of a geosynthetic (geogrid) reinforcement layer in a pullout box and (2) a geosynthetic (geotextile)-reinforced soil layer over a void. It also presents the results of parametric sensitivity analyses for the missing soil and interface model parameter values and identifies the values that give the best agreement with measured data. The paper further demonstrates that correct modeling of geometrical nonlinearity is key to accurately predicting the performance of geosynthetic-reinforced soil systems controlled by the tensioned membrane effect. The lessons learned in this study will be of interest to numerical modelers during numerical model design of geosynthetic-reinforced soil systems.