Yes, XMODEL can simulate nonlinear circuits/systems as well as linear circuits/systems in the same event-driven manner. For instance, XMODEL supports function primitives such as pwl_func, poly_func, sin_func, and exp_func that can perform nonlinear operations, and circuit primitives such as nmosfet, pmosfet, diode, and cap_var that represent nonlinear elements.
Nonlinear systems produce responses that are more diverse and complex than those of linear systems. XMODEL basically uses two methods to express such responses of nonlinear systems. One is to increase the number of terms in the signal’s functional expression and the other is to introduce additional events. Each has advantages and disadvantages and XMODEL uses the more effective one depending on the cases. For instance, XMODEL uses the former method for multiply primitive and the latter method for circuit primitives like nmosfet.
If you want to probe this topic further, you can refer to the academic papers listed below:

- J.-E. Jang, M.-J. Park, and J. Kim, “An Event-Driven Simulation Methodology for Integrated Switching Power Supplies in SystemVerilog,” ACM/IEEE Design Automation Conf. (DAC), June 2013.
- J.-E. Jang, S.-J. Yang, J. Kim, “Event-Driven Simulation of Volterra Series Models in SystemVerilog,” IEEE Custom Integrated Circuits Conf. (CICC), Sept. 2013.
- J.-E. Jang and J. Kim, “PPV-Based Modeling and Event-Driven Simulation of Injection-Locked Oscillators in SystemVerilog,” IEEE Trans. Circuits and Systems I, Aug. 2015.