How to address a knighted academic?
The canonical reference for this subject is Debrett's (via the Wayback Machine). They recommend that a letter to a knight should begin "Dear Sir [given name]," regardless of whether they have other titles such as "professor".

In fact, they advise against mixing titles granted by the sovereign (such as knighthoods and ranks in the armed forces) with titles not granted by the sovereign (such as professor) but, as others have observed, "Professor Sir [Given name] [Surname]" is common usage.

In an academic context, your alternative option  for a salutation is "Dear Professor [Surname]". However, as I noted in a comment to the question, it's unlikely that somebody who's the CEO of a multinational company would, in fact, be a professor.