  We present a parameterized model of the intra-cluster medium that is suitable
for jointly analysing pointed observations of the Sunyaev-Zel'dovich (SZ)
effect and X-ray emission in galaxy clusters. The model is based on assumptions
of hydrostatic equilibrium, the Navarro, Frenk and White (NFW) model for the
dark matter, and a softened power law profile for the gas entropy. We test this
entropy-based model against high and low signal-to-noise mock observations of a
relaxed and recently-merged cluster from N-body/hydrodynamic simulations, using
Bayesian hyper-parameters to optimise the relative statistical weighting of the
mock SZ and X-ray data. We find that it accurately reproduces both the global
values of the cluster temperature, total mass and gas mass fraction (fgas), as
well as the radial dependencies of these quantities outside of the core (r >
kpc). For reference we also provide a comparison with results from the single
isothermal beta model. We confirm previous results that the single isothermal
beta model can result in significant biases in derived cluster properties.
