Using React with webpack. Ran through some articles, but most of them suggest calling individual scss file for each component. But I would like to precompile all css into single file for entire application like we do using grunt/gulp.

You can use the webpack-text-extract-pluggin that is in charge of compiling all css files and bundling them in an index.css file.
Also note that you'll need to install sass-loader too in order to compile the scss.
In the webpack config:
var ExtractTextPlugin = require('extract-text-webpack-plugin');
config = {
    ..., 
    plugins: [
        ...,
        new ExtractTextPlugin('index.css')
    ],
    module: {
        loaders: [
            ...
            {
                test: /\.css$/,
                loader: ExtractTextPlugin.extract('style','css')
            },
            {
                test: /\.scss$/,
                loader: ExtractTextPlugin.extract('style', 'css!sass')
            }
        ]
    }
}
In index.html:
<link rel="stylesheet" type="text/css" href="/index.css">
In any Javascript file that gets through webpack:
require("./styles/my-custom-file.scss");
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