Write down the great idea for the new game on paper, spend about an hour or so jotting down all of the details and brainstorming all your ideas regarding it and then put it aside. Concentrate on your current project. You don’t want to handle things the way I did because that is just getting easily distracted by every idea and never getting anything finished at all.
Some people can juggle making more than one game at once. It’s probably best not to to begin with though.