From my book, it says that 'without van der Waals (London Dispersion) forces, it would be impossible to liquefy noble gases.'
Why is this the case?
I'd say that because noble gases are non-polar atoms, the only force that applies to them is the London force.
If you want a liquid phase, you need to have forces between the atoms / molecules that constitute this liquid (forces too strong would lead to solid state).
So because London force is the only one that applies on noble gases molecules, they are necessary to liquify to gases.