While cegaton's answer will work, I'd like to point out that your question is much more complex than what many folks might see at first glance and is a perfect example to examine the output view transform.

Photorealism and Flames

Flames are, as we all know, bright.

When we say a term such as "photorealistic" we can infer very quickly that the learned aesthetic is referencing something. In this case, it is referencing the learned aesthetic of photography.

Photographic still images hold a good deal of latitude, or the entire range of light from a scene. For example, if a particular film stock holds fifteen stops of light, it might mean that it holds 8 stops down and 7 stops above middle grey.

When we consider a flame, we are dealing with a very large ratio of light in the flame itself, possibly four or five stops or more over middle grey.

So what is happening in your image if you manage to get the emission correct on the candle?

The Secret Sauce of Views

The secret to this answer is the sRGB default view transform. This transform is directly dumped into the view via OpenGL calls, which are clipped at a display referred value of typically 1.0. That is, the sRGB view LUT is completely blind and ignorant, and chops off all of the scene referred data above 1.0.

In your scene, the linear value of 0.2 will be your baseline middle grey via the default view transform. That leaves a meager two and a bit stops above middle grey for the entire scene's highlights. That is, 0.2 + 0.2 is one stop, 0.4 + 0.4 is two stops, and the remaining 0.2 is a sliver of the next stop.

When we compare that with the amount of light our sample film stock holds, we can see that the round off of highlights is entirely chopped off, and our candle flame bursts out to display referred white long before an equivalent film stock would.

Reconciling Display Referred and Scene Referred

The whole concept of photorealism then, is inherently broken, and fundamentally impossible, with the default sRGB display referred transform from the scene referred data.

As a result, another possible solution could be to apply a method to get more stops into a gentle roll-off of the default sRGB view transform. In this case, trying a tone mapping operation or using a custom curve in the Color Management settings that bends roughly six or seven stops of latitude into the range between 0.2 and 1.0 display referred.

Again, cegaton's answer will obviously work fine, but this question provides a wonderful opportunity to explore the nature of "photorealism" and the OpenColorIO view transformations.