  An elementary proof is given for the existence of infinite dimensional
abelian subalgebras in quantum W-algebras. In suitable realizations these
subalgebras define the conserved charges of various quantum integrable systems.
We consider all principle W-algebras associated with the simple Lie algebras.
The proof is based on the more general result that for a class of vertex
operators the quantum operators are related to their classical counterparts by
an equivalence transformation.
