Why naturalism excludes the supernatural

The problem with naturalism excluding the supernatural is that, at least in one sense, it's trivially true. Of course if everything is natural, then the supernatural is excluded. But that doesn't tell me, by itself, what is natural and what is supernatural. If God could be a theoretical entity in a scientific explanation, then we could say that whatever appears in a scientific explanation is natural, and therefore God is a natural entity. If you say God can't be a theoretical entity in a scientific explanation, then you have to come up with characteristics of God that require God's exclusion.