Related question (The system is not limited to integrable model, so I think this question is different)
As far as I know in quantum mechanics, exact solutions for multivariable systems (from partial differential equation, than ordinary differential equation), e.g., the hydrogen atom and Hooke atom, are obtained from separation of variable method. Is there any multivariable example in physics (doesn't have to be quantum), the exact solution can be obtained without using separable of variable method?