Why do we consider all gasses to be in one phase? Why do we take the phase of all gases 1. If all gases exist in 1 phase then why do we say that some gases are heavier than air, some are light also we collects some gases in lab by air displacement method. Is this all possible if gases are completely miscible.
Just because two fluids are completely miscible does not guarantee that they are uniformly mixed at all times.  

For example, if you have a glass half full with sugar water solution, you can gently pour pure water to form a layer on top.  This is the basis for sucrose density gradient centrifugal separation and layered drinks.

So the same can occur for gases, a non-equilibrium state can exist for considerable time where the gases aren't mixed even though they are completely miscible.