  Finite volume multiple-particle interaction is studied in a two-dimensional
complex $\phi^4$ lattice model. The existence of analytical solutions to the
$\phi^4$ model in two-dimensional space and time makes it a perfect model for
the numerical study of finite volume effects of multi-particle interaction. The
spectra from multiple particles are extracted from the Monte Carlo simulation
on various lattices in several moving frames. The $S$-matrix of multi-particle
scattering in $\phi^4$ theory is completely determined by two fundamental
parameters: single particle mass and the coupling strength of two-to-two
particle interaction. These two parameters are fixed by studying
single-particle and two-particle spectra. Due to the absence of the diffraction
effect in the $\phi^{4}$ model, three-particle quantization conditions are
given in a simple analytical form. The three-particle spectra from simulation
show remarkable agreement with the prediction of exact solutions.
