In deep North Sea reservoirs, condensate gases have been found at high temperatures (up to 190 degrees C) and pressures (up to 1100 bar). Some of these methane-rich fluids are near critical and may contain significant amounts of high molecular weight hydrocarbons. These features make it particularly difficult to study their thermodynamic behaviour, as well from an experimental as from a theoretical point of view. As such contrasted mixtures have not been extensively studied in the literature, we have also started data acquisition on synthetic mixtures. Compared with real fluids, synthetic mixtures allow indeed a more reliable test of thermodynamic models because their composition is much better controlled. Four synthetic gas condensates containing 6 or 7 components have been investigated in a visual cell to show the sensitivity of phase equilibria with respect to small quantities of heavy alkanes (nC(36)) and aromatics (phenanthrene). A very large sensitivity has been found, since addition of about 1% mel. of a heavy hydrocarbon may increase dew pressures by 200 bar in some cases. Crystallization of heavy hydrocarbons has been observed for temperatures 10-30 degrees C lower than pure component melting temperatures. These features have been modelled, using the Peng-Robinson equation of state for fluid phases. As a general feature, the Peng-Robinson EOS reproduces adequately the phase envelope of these fluids with regressed interaction parameters between methane and the heaviest hydrocarbon. However, prediction of liquid dropout is unsatisfactory. A simple model of crystallization has been used to Predict appearance of solid from gas, which accounts for solid state transitions. This model accounts fairly well for phenanthrene or nC(36) crystallization at high pressure.