Preheat a grill or grill pan over medium-high heat. Grill the corn for 15 minutes, rotating every 5-7 minutes, until golden brown on most of the ears. Remove from the grill and set aside to cool a few minutes. You can prepare the rest of your salad ingredients while the corn cools.
In a large bowl, combine the radish slices, scallions, cilantro, and chili powder. Juice the lime into the bowl.
When the corn is cool enough to handle, carefully cut the kernels off of the cobs. Combine the corn kernels with the sliced radish mix. Toss well, taste to adjust seasoning, and serve.