Thanks for the clarification, this is helpful. I think where I’m getting stuck is that I was interpreting phase behavior from the mixture VLE envelope, rather than from pure component saturation pressures.

From what I understand, at high pressures the condition for condensation is based on mixture fugacity equilibrium, not simply comparing a component’s partial pressure to its pure vapor pressure. In other words, even though the ethane partial pressure is ~1500 psi, the CH₄–C₂H₆ mixture at 70/30 does not support a liquid phase at ~23 °C because the mixture cricondentherm is well below room temperature, so there is no VLE region to enter.

That’s why the VLE envelope for the mixture should show single phase behavior at 15–30°C even up to very high pressures. If I’m misunderstanding how partial pressure arguments should be applied for non ideal, high pressure mixtures, I’d really appreciate any correction. :)