User blog comment:Uraharattack/Chapter 464: 400 Years (It's out guys)/@comment-26537859-20151130154544/@comment-124.82.58.141-20151202072030

I don't think Natsu is using Igneel's power prior to facing Zeref. It's a limited power. Also if he's using it, then people would be bound to notice the change in magic, even if it's only for a short while. Plus the effects of the flame before & after Natsu enters the Dragon King mode is obvious too. I believe we should conclude that Natsu has been fighting with his own powers instead of Igneel's power.