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Here's how it's done:

require 'radiator'

comment = {
  type: :comment,
  parent_permlink: 'test',
  author: 'your-account',
  permlink: 'something-unique',
  title: 'Radiator Can Post Comments!',
  body: 'Yep, this post was created by Radiator in `ruby`.',
  json_metadata: '',
  parent_author: ''
}

tx = Radiator::Transaction.new(wif: 'Your Wif Here')
op = Radiator::Operation.new(comment)
tx.operations << op
tx.process(true)